A convicted murderer in the United States struggled and gasped for air for at least 10 minutes in a prolonged execution using a controversial new drug cocktail, witnesses say.

Dennis McGuire, 53, was injected with sedative midazolam mixed with painkiller hydromorphone, a combination never previously used in an execution in the United States.

McGuire, who was sentenced to death in 1989 for the rape and murder of a young pregnant woman, made "snorting and choking sounds" as he died, according to a journalist for the Columbus Dispatch newspaper present at the execution.

The new execution protocol was introduced after Ohio and other US states that retain the death penalty began running out of barbiturates when European manufacturers stopped supplying them.

McGuire's lawyers had opposed the method of execution, saying the killer would die of asphyxia in a phenomenon known as "air hunger", inflicting the sort of cruel and unusual punishment prohibited under the US Constitution.

But appeals, which went all the way to the US Supreme Court, were rejected.

A federal judge in Ohio, Gregory Frost, said "the evidence before the court failed to present a substantial risk that McGuire will experience severe pain."

According to reporters at the execution, McGuire's execution was the longest since Ohio re-introduced the death penalty in 1999.

"At about 10:33am, McGuire started struggling and gasping loudly for air, making snorting and choking sounds that lasted for at least 10 minutes, with his chest heaving and his fist clenched," the Columbus Dispatch reported.

"Deep, rattling sounds emanated from his mouth.

"For the last several moments before he was pronounced dead, he was still."

McGuire was pronounced dead at 10:53am (local time), the Ohio Department of Rehabilitation and Correction said in a statement.

Execution 'an experiment that failed': McGuire's lawyer

Afterwards, attorney Allen Bohnert, an assistant federal public defender who represented McGuire, urged Ohio governor John Kasich to declare a moratorium on executions.

"The experiment that was this execution has failed," Mr Bohnert said.

US district judge Gregory Frost refused to stop the execution, but ordered prison officials to preserve the medication vials, the boxes in which they were packaged and the syringes used for McGuire's execution.

McGuire's execution was the third time Ohio has used untested execution drugs.

Three botched executions from 2006 to 2009 had resulted in Frost ruling Ohio's lethal injection protocols unconstitutional and ordering a temporary halt to executions.

Death penalty experts voiced concern at the details of McGuire's death, saying it pointed to a method of execution which was "egregious and problematic".

"In light of the length and disturbing descriptions of Dennis McGuire's execution, in addition to the range of lethal injection complications reported in other states, it appears that this country's lethal injection procedure is more egregious and problematic than it ever has been," Deborah Denno from the Fordham University School of Law said.

The execution was the second carried out this year by US authorities using new products that appeared to result in suffering of the condemned.

On January 9, in Oklahoma, condemned killer Michael Lee Wilson said he could feel his "whole body burning" as he was put to death.

Wilson was executed using a mix of drugs including pentobarbital, a substance commonly used to euthanise animals.

McGuire is the third man to be executed in the United States this year. Ohio executed three of the 39 people put to death in the United States last year, according to the Death Penalty Information Center.